1. Field of the Invention
This invention relates to a etching process for the removal of oxide and/or silicon residues during the formation of integrated circuit structures on semiconductor wafers. In particular, this invention relates to a dry process for the removal of materials from the sidewalls of raised lines on semiconductor wafers using a high pressure magnetically enhanced plasma etch and a gaseous etchant containing NF.sub.3 gas.
2. Description of the Related Art
During the construction of integrated circuit structures on and in a semiconductor wafer, undesirable oxide and/or silicon products or residues may be formed or left on the wafer. For example, during the formation of conductive polysilicon lines on the surface of a semiconductor wafer, such undesirable products or residues may be formed or left on the sidewalls of the resulting polysilicon line or lines.
For example, as shown in FIG. 1, when a polysilicon line 10 is formed over a gate oxide layer 6 on a silicon semiconductor wafer 2, by patterning a photoresist layer to form a photoresist mask 14 and then anisotropically etching an underlying polysilicon layer through the mask, a residue 20a may form on the sidewalls of polysilicon line 10, and a similar residue 20b may form on the sidewalls of photoresist mask 14. Such residue material is believed to be a form of a silicon-rich oxide, which is sometimes denoted as a fluffy oxide because of its appearance under a microscope due to its relative density when compared to thermal oxide and its etch rate in hydrofluoric acid. This residue material must be removed from the sidewalls, for example, when it is desired to form oxide sidewalls for insulation purposes, because it interferes with high quality oxidation processes since it protrudes above the poly line itself. This residue also creates adhesion problems for the next layer and can contaminate the diffusion furnaces in the next level processing if not removed.
In the past, removal of this oxide residue has been carried out by a wet etch process using HF. However, this necessitates removal of the wafer from a vacuum apparatus, which can introduce contamination, as well as slow down the throughput of the overall process for forming integrated circuit structures on the wafer. Furthermore, the use of HF will also attack the gate oxide, thus requiring very close monitoring of the wet etch step.
Oxide type residues are not the only form of residues which may be formed on the sidewalls of polysilicon lines on semiconductor wafers. For example, as shown in FIGS. 2-4, when forming a polysilicon line 30 on semiconductor wafer 2 over an oxide layer 26 which separates polysilicon line 30 from an underlying polysilicon line 22, the normal anisotropic etching of the upper polysilicon layer through a photoresist mask 36 can result in polysilicon residues or stringers 40 remaining on oxide layer 26 at 28 where oxide layer 26 necks down as it is formed over the sidewalls of lower polysilicon line 22. Such polysilicon stringers 40, if not removed, can form an undesirable short or conductive paths between two parallel polysilicon lines 30 formed over oxide layer 26, as best seen in FIG. 3.
As in the removal of the oxide residues, the current practice used to remove such undesirable polysilicon stringers is to remove the wafer from the vacuum apparatus and to subject it to a wet etch, usually a KOH etch, to remove the undesirable polysilicon.
However, when an etchant such as KOH is used to remove such undesirable polysilicon residues, great care must again be exercised in controlling the etch since the etchant will also attack the polysilicon line. In addition, in submicron geometries, wet chemicals may have problems penetrating high aspect ratio openings.
It would, therefore, be desirable to provide a dry etch process which could selectively remove such undesirable residues without the need to remove the semiconductor wafer from the vacuum chamber.